1. Field of the Invention
The present invention relates to a hologram laser unit and an optical pickup apparatus, which are preferably used when reading information of an optical recording medium such as a CD (Compact Disk) and a DVD (Digital Versatile Disk) and recording information onto the optical recording medium.
2. Description of the Related Art
An optical pickup apparatus is used for reading and recording information from and onto an optical disk-shaped recording medium (hereinafter, simply referred to as an ‘optical recording medium’). Since before, an optical recording medium called a CD (Compact Disk) family which information is read from and written in by the use of only light has been used, and at the time of reading and recording information from and onto this optical recording medium, a semiconductor laser element which emits a laser light beam of an infrared wavelength whose oscillation wavelength is 780 nm is used.
In recent years, an optical recording medium called a DVD (Digital Versatile Disk) family which information is read from and written in by the use of light and which allows recording of more information than the CD family also comes to be used in large quantities, and at the time of reading and recording information from and onto this optical recording medium, a semiconductor laser element which emits a laser light beam of an infrared wavelength whose oscillation wavelength is, for instance, 630 nm or more and less than 690 nm is used. Therefore, an optical pickup apparatus which is capable of reading and recording information from and onto both the optical recording mediums of the CD family and the DVD family is demanded, and is being developed.
In a first related art, an optical pickup apparatus is provided with two light sources which emit laser light beams of different oscillation wavelengths and one hologram element designed so that the efficiency of light use of a laser light beam of a short oscillation wavelength becomes large, and structured so as to be capable of finely playing back both an optical recording medium of relatively high recording density to be played back by the use of a laser light beam of a short wavelength, such as a DVD, and an optical recording medium of relatively low recording density to be played back by the use of a laser light beam of a long wavelength, such as a CD (refer to, for instance, Japanese Unexamined Patent Publication JP-A 9-73017 (1997)).
In a second related art, a laser module in which two semiconductor laser diodes whose oscillation wavelengths are different from each other and an optical element for condensing laser light beams emitted from the semiconductor laser diodes, respectively, onto an information recording surface of an optical recording medium are integrated in a single body, whereby reproduction and record of information from and onto plural standards of optical recording mediums is enabled (refer to, for instance, Japanese Unexamined Patent Publication JP-A 9-120568 (1997)).
In a third related art, a first semiconductor laser element which emits a laser light beam whose oscillation wavelength is 650 nm, a second semiconductor laser element which emits a laser light beam whose oscillation wavelength is 780 nm and a light receiving element are installed in a single package. A first light transmitting substrate is mounted above the package, and a three-beam diffraction grating and a hologram element for diffracting only the laser light beam emitted from the first semiconductor laser element are formed on the first light transmitting substrate. Moreover, a second light transmitting substrate is mounted above the first light transmitting substrate, and a hologram element for diffracting only the laser light beam emitted from the second semiconductor laser element is formed on the second light transmitting substrate. Light obtained when the laser light beam emitted from the first semiconductor laser element is reflected by an optical recording medium is diffracted and guided to the light receiving element by the hologram element on the second light transmitting substrate, and light obtained when the laser light beam emitted from the second semiconductor laser element is reflected by an optical recording medium is diffracted and guided to the light receiving element by the hologram element on the first light transmitting substrate (refer to, for instance, Japanese Unexamined Patent Publication JP-A 2000-76689).
In a fourth related art, an optical pickup apparatus is provided with a first hologram which has a first hologram diffraction grating formed on a surface thereof, and a second hologram which has a second hologram diffraction grating formed on a surface thereof and is mounted on the first hologram so as to cover the first hologram diffraction grating. The surface area of the first hologram on a side of the second hologram is larger than the surface area of the second hologram on a side of the first hologram.
When the second hologram is mounted on the first hologram, firstly, in a position on the surface of the first hologram which corresponds to each apex of the second hologram on the first hologram side, the second hologram is placed after an ultraviolet cure resin (abbreviated as a UV resin) is dropped, and temporarily secured by irradiating the UV resin with an ultraviolet ray after an optical adjustment. Secondly, the UV resin is applied to a portion not in contact with the second hologram of the surface of the first hologram and a lower portion of a side surface of the second hologram, and the UV resin is irradiated with the ultraviolet ray, whereby the second hologram is secured on the first hologram (refer to, for instance, Japanese Unexamined Patent Publication JP-A 2002-72143).
In a fifth related art, a first hologram substrate and a second hologram substrate are disposed in a single body. The first and second hologram substrates have a focus detecting hologram portion and a track detecting strip hologram portion. After the second hologram substrate is mounted on the first hologram substrate, and an optical axis adjustment and an offset adjustment are performed, the first hologram substrate and the second hologram substrate are adhered and secured by an adhesive to become a single body. At this moment, the adhesive is applied to portions through which a laser light beam emitted from a light source does not pass of the first and second hologram substrates and a side surface of the second hologram substrate, whereby the first hologram substrate and the second hologram substrate are adhered to become a single body (refer to, for instance, Japanese Unexamined Patent Publication JP-A 2002-279683).
In the aforementioned third to fifth related arts, two semiconductor laser elements are placed adjacent to each other in a position such that optical axes of laser light beams emitted from the respective semiconductor laser elements become nearly coincide so that the laser light beams of different oscillation wavelengths emitted from the two semiconductor laser elements, respectively, enter both first and second hologram elements. Therefore, resulting from the incidence of the laser light beams which are emitted from the respective semiconductor laser elements and reflected by the optical recording medium so as to be diffracted by the first hologram element, into a part of or whole second hologram element, unnecessary light is generated. Moreover, there arises a problem such that the amount of laser light beams which should be condensed onto an optical recording element decreases, whereby the efficiency of the use of light lowers and reliability lowers. In order to solve these problems, it is required to space the two hologram elements out so that the laser light beam diffracted by the first hologram element does not interfere in the second hologram element. In order to arrange the two hologram elements as above, there is a problem such that the optical pickup apparatus becomes large because it is required to enlarge the dimension in a thickness direction of the substrate on which the respective hologram elements are formed.
In order to solve the above-mentioned problem such that the efficiency of the use of light lowers, it is required to make the dimensions in thickness directions of diffraction grating grooves formed on the three-beam diffraction grating and the second hologram element to be dimensions such that only a laser light beam emitted from the second semiconductor laser element is diffracted, and make the dimension in a thickness direction of a diffraction grating groove formed on the first hologram element to be a dimension such that only a laser light beam emitted from the first semiconductor laser element is diffracted. However, since pitches of the diffraction gratings of the first and second hologram elements are small as compared with that of the three-beam diffraction grating, there is a problem such that it is difficult to provide the first and second hologram elements with diffraction grating grooves having dimensions such that only one of the laser light beams emitted from the two semiconductor laser elements is diffracted.